Which of the following shows the correct order of decreasing stability?

Options

A.
B.
C.
D.

Answer

To determine the correct order of decreasing stability among the given carbocations, we need to analyze the stabilization mechanisms for each type of carbocation presented in the options. Here's a step-by-step solution:

### Step 1: Identify the Carbocations
We have four different carbocations to compare. Let's denote them based on the provided information:
1. **Carbocation A**: Benzyl carbocation with a methoxy group (OCH₃).
2. **Carbocation B**: Methyl carbocation (CH₃).
3. **Carbocation C**: Ethyl carbocation (C₂H₅).
4. **Carbocation D**: Simple benzyl carbocation (C₆H₅CH₂⁺).

### Step 2: Analyze Stabilization Effects
- **Carbocation A (Benzyl with Methoxy)**: The methoxy group (OCH₃) can donate electron density through resonance (+R effect), significantly stabilizing the carbocation. This resonance effect is very strong and provides substantial stabilization.

- **Carbocation B (Methyl)**: The methyl group can stabilize the carbocation through hyperconjugation. While this effect is helpful, it is not as strong as resonance.

- **Carbocation C (Ethyl)**: The ethyl group also stabilizes the carbocation through hyperconjugation, similar to the methyl group but slightly less effective due to the larger size and lesser electron-donating ability compared to a methoxy group.

- **Carbocation D (Benzyl)**: The benzyl carbocation is stabilized by resonance as well, but it does not have the additional stabilization from the methoxy group. It is less stable than the carbocation with the methoxy group.

### Step 3: Compare Stability
Based on the stabilization effects:
- **Carbocation A** (Benzyl with Methoxy) is the most stable due to strong resonance stabilization.
- **Carbocation D** (Benzyl) is next, as it has resonance stabilization but lacks the electron-donating methoxy group.
- **Carbocation B** (Methyl) comes next due to hyperconjugation.
- **Carbocation C** (Ethyl) is the least stable, as it relies solely on hyperconjugation.

### Step 4: Establish the Order of Stability
Thus, the order of decreasing stability for the carbocations is:
1. **Carbocation A** (Benzyl with Methoxy)
2. **Carbocation D** (Benzyl)
3. **Carbocation B** (Methyl)
4. **Carbocation C** (Ethyl)

### Final Answer
The correct order of decreasing stability is: **Carbocation A > Carbocation D > Carbocation B > Carbocation C**.